1. Field of the Invention
The present invention relates to a standard solution for simultaneously calibrating a plurality of electrodes for determining the concentrations of a plurality of ions (including at least potassium ions and calcium ions) in blood.
2. Description of the Prior Art
In general, magnesium ions, iron ions, copper ions and the like having an ionic strength of about 150 mM are contained in blood in addition to sodium ions (hereinafter referred to as Na.sup.+) of 135 to 145 mM, potassium ions (hereinafter referred to as K.sup.+) of 3.5 to 5.0 mM and calcium ions (hereinafter referred to as Ca.sup.++) of 1.2 to 1.6 mM. In addition, the pH of blood is 7 to 7.5. Where the concentrations of said ions contained in blood are determined by means of electrodes, it is necessary to calibrate the electrodes periodically by the use of a standard solution which possesses a definite known value for the electrodes, respectively.
When measuring a blood example, it is desirable to carry out the calibration of said electrodes near the ion strength of the ions in the blood sample. The electrodes determine the ionic activity expressed as the product of the concentration and the activity coefficient, which is dependent upon the concentration of the ion to be determined, as well as the concentration of other ions contained in the blood. However, the activity coefficients of ions having equal ion strengths are equal to each other, regardless of the other kinds of ions contained in the blood. Accordingly, where the concentrations of Na.sup.+, K.sup.+ and Ca.sup.++ contained in the blood are determined, it is desirable to calibrate the electrodes by the use of a standard solution of which ion strength is set to about 150 mM.
Although standard solutions for individually calibrating electrodes for determining the Na.sup.+, K.sup.+, Ca.sup.++ or pH is on the market, a standard solution capable of simultaneously calibrating the measurement electrodes corresponding to the above described parameters has never yet been realized. For example, a standard solution merely used for simultaneously calibrating a Ca.sup.++ electrode and a pH electrode is disclosed in Japanese Patent Publication unexamined application No. 35062/1981.